1. Field of the Invention
The present invention relates to a covering material for an electric wire and more particularly to a polyester resin composition comprising a halogen-containing flame retardant aromatic polyester copolymer and a bisepoxy compound incorporated therein. The covering material of the present invention does not suffer any flexibility loss due to thermal history and has excellent flame retardancy. The present invention also relates to an electric wire covered with the same.
2. Prior Art
Rubber, polyvinyl chloride, polyethylene, polypropylene, nylon, etc. have hitherto been used as a wire covering material. In particular, polyvinyl chloride has extensively been used for this application from the viewpoint of flame retardancy and mechanical strengths. In recent years, with an increase in the severity of the environment under which the above-described covering materials are used, requirements for the properties of the covering material including not only excellent thermal resistance and electrical characteristics but also flame retardancy and an excellent thin-wall forming property necessary for realizing space saving have been increased.
Although fluororesin, crosslinked polyethylene, etc., can meet the above-described requirements, they are unsatisfactory in that they are both poor in the thin-wall forming property and the fluororesin is expensive.
Attention has been paid to polyethylene terephthalate and polybutylene terephthalate because they are excellent in the thin-wall forming property as well as in the mechanical strengths (flexibility, abrasion resistance, etc.), thermal resistance, and electrical characteristics. However, these terephthalates are insufficient in the flame retardancy. Further, since they are crystalline resins, the heat history including heating treatment after covering a wire and heating during the use of the covered wire bring about a remarkable lowering in the flexibility thereof. This in turn brings about a lowering in the mechanical strengths such as impact resistance. For this reason, the use of the terephthalates in the vicinity of heat sources and in environments having a danger of heat buildup should be avoided, which more or less impose restrictions on the use of these materials.
In order to overcome the above-described drawbacks, attempts have been made lower the crystallinity even to a small extent by the addition of an elastomer by partial crosslinking for the purpose of maintaining the stability of the mechanical strengths.
The former expedient brings about some improvement, but is disadvantageous in that it is impossible to withstand long-term heat history since the crystalline resin matrix remains as is and, further, the lowering in the proportion of the crystalline resin brings about a lowering in the mechanical properties such as abrasion resistance.
The latter expedient brings about some improvement in the stability of the mechanical strengths, but is disadvantageous in that the flexibility is spoiled and the fabricability is remarkably lowered because of the complexity in controlling the crosslinking reaction.